1. Field of the Invention
The present invention relates to a negative-electrode active material, and a method for the production of a negative-electrode active material.
2. Description of Related Art
With the recent rapid spread of information and communication devices such as personal computers, video cameras and cellular phones, the development of batteries that are used as power sources for the devices is regarded as important. In the automotive industries, high-output and high-capacity batteries for electrical or hybrid vehicles are under development. Among various types of batteries, lithium batteries are attracting attention because of their high energy density and output.
A lithium battery usually includes a positive-electrode active material layer that contains a positive-electrode active material, a negative-electrode active material layer that contains a negative-electrode active material, and an electrolyte layer that is interposed between the electrode active material layers, and optionally includes a positive electrode current collector that collects current from the positive-electrode active material layer and a negative electrode current collector that collects current from the negative-electrode active material layer.
Well-known examples of the negative-electrode active material include carbon-based negative-electrode active materials and metal-based negative-electrode active materials. In general, metal-based negative-electrode active materials have an advantage of having a higher theoretical capacity than carbon-based negative-electrode active materials. On the other hand, the problems of metal-based negative-electrode active materials are that they significantly change in volume in response to insertion and extraction of lithium ions as conductive ions and tends to cause cracks in the negative-electrode active material layer or layers adjacent to the negative-electrode active material layer and flaking or powdering of the negative-electrode active material, and that they are poor in cycle characteristics. For example, the theoretical capacity of silicon (Si) is approximately 4200 mAh/g, which is more than ten, times that of carbon, which is approximately 372 mAh/g. Thus, studies are being conducted for a silicon-based negative-electrode active material that contains silicon which shows significant improvement in various properties such as cycle characteristics (for example, Japanese Patent Application Publication No. 6-325765 (JP 6-325765 A), Japanese Patent Application Publication No. 2008-198610 (JP 2008-198610 A), Japanese Patent Application Publication No. 2009-70825 (JP 2009-70825 A), Japanese Patent Application Publication No. 2010-140901 (JP 2010-140901 A), Japanese Patent Application Publication No. 2009-164104 (JP 2009-164104 A), Japanese Patent Application Publication No. 2004-349237 (JP 2004-349237 A)).
For example, JP 6-325765 A discloses a lithium secondary battery that uses a lithium-containing silicon oxide or silicate which can occlude and release lithium as a negative-electrode active material.